The rapid spread of optical communication systems has increased the need for proper security solutions, in order to prevent eavesdropping and jamming. Existing Spread Spectrum encryption techniques are considered to be appropriate for optical communication systems, due to their large bandwidth which enables to achieve high processing gain [1]. In addition, methods for encrypting transmission using Optical Coherent Optical Code-Division Multiple-Access (OCDMA) are proposed in [2-4]. According to these methods, the signal is encrypted in the time domain using a dispersive element which encodes the spectral phase of the signal. By implementing the approaches presented in [5,6], the temporal phase of the dispersed signal is encoded using a Temporal Phase Encoder (TPE), thereby assigning different phase to each spectral component. However, by using these encryption methods while transmitting outside the bandwidth of a public channel, the spectral concealment of the signal is not always assured since, the signal is not necessarily spectrally hidden under the noise level. If the signal is not concealed in the frequency domain, an adversary that coherently detects and samples the signal can perform Discrete Fourier Transform (DFT) and disclose the spectral amplitude of the signal. Therefore, the transmitted signal is not spectrally stealthy.
All the methods described above failed to propose a method and system for providing stealthy fiber optic communication, which prevents eavesdropping and jamming both in spectral and temporal domains.
It is an object of the present invention to provide a method that allows transmitting a signal with reduced power spectral density.
It is another object of the present invention to provide a method that allows transmitting a signal, while keeping it below the noise level in the frequency domain.
Other objects and advantages of the invention will become apparent as the description proceeds.